Along with the miniaturization of semiconductor devices, the trend is moving into shorter wavelength of the exposure light source and higher numerical aperture (high NA) of the projection lens. At present, an exposure machine with NA of 0.84 has been developed, where an ArF excimer laser having a wavelength of 193 nm is used as the light source. As commonly well known, the resolving power and the focal depth can be expressed by the following formulae:(Resolving power)=k1·(λ/NA)(Focal depth)=±k2·λ/NA2 wherein λ is the wavelength of the exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are constants related to the process.
In order to realize still shorter wavelength and higher resolving power, studies are being made on an exposure machine where an F2 excimer laser having a wavelength of 157 nm is used as the light source. However, the lens material used for the exposure apparatus so as to realize shorter wavelength and the material used for the resist are very limited and therefore, it is extremely difficult to stabilize the production cost or quality of the apparatus and materials. This may lead to a failure in procuring the exposure apparatus and the resist each assured of sufficiently high performance and stability within a required time period.
Conventionally, a so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample has been known as a technique of increasing the resolving power in an optical microscope.
As for the “effect of immersion”, assuming that the wavelength of exposure light in air is λ0, the refractive index of the immersion liquid to air is n, the convergence half-angle of beam is θ and NA0=sin θ, the above-described resolving power and focal depth when immersed can be expressed by the following formulae:(Resolving power)=k1·(λ0/n)/NA0 (Focal depth)=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system with the same NA, the focal depth can be made n times larger by the immersion.
This is effective for all pattern profiles and can be combined with super-resolution techniques such as phase-shift method and modified illumination method which are being studied at present.
The apparatus where this effect is applied to the transfer of a fine pattern of a semiconductor device is described, for example, in JP-A-57-153433 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-7-220990, but these references are silent on the resist suitable for the immersion exposure technique.
JP-A-10-303114 indicates that change in the refractive index of immersion liquid brings about deterioration of the projected image due to wavefront aberration of the exposure machine and therefore, the control of refractive index of the immersion liquid is important, and discloses a technique of controlling the temperature coefficient of refractive index of the immersion liquid to a certain range or using, as a suitable immersion liquid, water where the surface tension is decreased or an additive for increasing the surface activity is added. However, this reference does not disclose the additive or is also silent on the resist suitable for the immersion exposure technique.
Recent progress of the immersion exposure technique is reported, for example, in Proceedings of Society of Photo-Optical Instrumentation Engineers (Proc. SPIE), Vol. 4688, page 11 (2002) and J. Vac. Sci. Tecnol. B, 17 (1999). In the case of using an ArF excimer laser as the light source, in view of safety on handling as well as transmittance and refractive index at 193 nm, pure water (refractive index at 193 nm: 1.44) is considered to be a most promising immersion liquid.
In the case of using an F2 excimer laser as the light source, a fluorine-containing solution is being studied in the light of balance between transmittance and refractive index at 157 nm, but those satisfied in view of environmental safety or refractive index have been not yet found out. Considering the degree of immersion effect and the maturity of resist, the immersion exposure technique is expected to be most soon mounted on an ArF exposure machine.
Since the discovery of a resist for a KrF excimer laser (248 nm), an image forming method called chemical amplification is used as the image forming method for a resist so as to compensate the reduction in the sensitivity due to light absorption. The image forming method, for example, using positive chemical amplification is an image forming method where an acid generator in the exposed area decomposes upon exposure to generate an acid, the acid generated is used as a reaction catalyst in the baking after exposure (PEB: post exposure bake) to convert the alkali-insoluble group into an alkali-soluble group, and the exposed area is removed by an alkali developer.
In the immersion exposure, the resist film is exposed through a photomask in the state of an immersion liquid being filled between the resist film and the optical lens to thereby transfer the pattern of the photomask to the resist film, and it is estimated that the immersion liquid permeates into the inside of the resist film and affects the resist performance.
When a chemical amplification resist is applied to the immersion exposure technique, the acid on the resist surface generated upon exposure moves to the immersion liquid and the acid concentration on the surface of exposed area is changed. This may be considered to have a close resemblance to the acid deactivation occurring on the surface of exposed area due to basic contamination in a very small amount on the order of several ppb mingled from the environment at the time delay between exposure and PEB (PED: post-exposure time delay), which is a serious problem at the initiation of development of the chemical amplification-type positive resist, but the effect of immersion exposure on the resist or the mechanism thereof is not clearly known.
On the other hand, it has. been found that when a chemical amplification-type resist causing no problem in the lithography by normal exposure is used for the pattern formation by the immersion exposure, there arises a problem such as generation of development defect or development residue (scum) or elution of the resist into the immersion liquid.